After you grab the blue key check the wall to your left to find a switch (linedef #2709), it will lower the pillar with the supercharge. Strafe jump to the lowered platform from opposite side you are currently on. (sector 348)

After you grab the blue key to the right is a wall with a misaligned texture (linedef #2777). Use it reveal a rocket launcher and a box of rockets. (sector 378)